A conventional engine starter for use in staring up a vehicular engine has been constructed as shown in FIG. 1.
The conventional engine starter 1 shown in FIG. 1 comprises a d.c. motor 2, an over-running clutch mechanism 4 slidably mounted over an extended shaft portion of an armature rotary shaft 3, a solenoid switch assembly 5 disposed on one side of the d.c. motor 2 for slidably moving the over-running clutch mechanism 4 along the extended shaft portion, and a shift lever 7 engaged at its one end with a plunger rod 8 of the solenoid switch assembly 5 and at its the other end with an annular member 6 mounted to the over-running clutch mechanism 4.
Thus, the conventional engine starter 1 needs the shift lever 7 for sliding the over-running clutch mechanism 4 on the extended shaft portion of the armature rotary shaft 3, and since the bi-axial arrangement, in which the solenoid switch assembly 5 for actuating the shift lever 7 and supplying electric power to the d.c. motor 2 is disposed on the side of the d.c. motor 2, is employed, the engine layout during the design of the vehicle is severely limited.
In order to eliminate the above problem, it has been proposed to position the solenoid switch on one of the axial end portions of the d.c. motor to form the starter into a simpler configuration such as an elongated cylinder. According to this proposition, the basic construction is such that the armature rotary shaft is made hollow and that the plunger rod of the solenoid switch assembly which has been used to operate the shift lever in the conventional design is elongated to extend through the inner passage or a central bore of the armature Electromagnetic core to reach to the output rotary shaft, and the thus-constructed starter is referred to as a coaxial starter since the armature rotary shaft of the d.c. motor and the rod of the solenoid switch are axially aligned. In order to further reduce the axial length of the coaxial starter having the above basic structure, a starter has also been proposed in which an over-running clutch mechanism is provided within the interior of the armature rotary shaft constituting the armature of the d.c. motor.
The engine starter containing the over-running clutch mechanism has formed in the inner surface of the armature rotary shaft recessed portions in which a plurality of cam surfaces are formed to define wedgeshaped spaces between the cam surfaces and the outer surface of a clutch inner member inserted within the armature rotary shaft. Each of the wedge-shaped spaces is provided with a roller and a biasing spring. In other words, the starter of this kind is constructed such that the armature rotary shaft is also used as a clutch outer member of the over-running clutch.
However, in the engine starter constructed as described above, the armature rotary shaft has formed on its outer circumferential surface a plurality of axially extending grooves and ridges such as serrations, which are formed generally by rolling. During the rolling, the armature rotary shaft must be pressed hard against a rolling tool, so that the clutch cam surfaces formed in the inner circumferential surface of the hollow armature rotary shaft are disadvantageously deformed.